Heat transfer tube and heat exchanger using same

ABSTRACT

A heat transfer tube is formed with a corrugated water tube to be used in a heat exchanger, and satisfying 0.04≦Hc/OD, where Hc is the corrugated groove depth of the corrugated tube and OD is the corrugation outside diameter thereof.

The present application is based on Japanese patent application No.2006-038531, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heat transfer tube and a heatexchanger using the heat transfer tube, and particularly, to a heattransfer tube for a water-refrigerant heat exchanger used in a naturalrefrigerant heat pump water heater (which may herein be referred to assimply “heat pump water heater”), and a heat exchanger using the heattransfer tube.

2. Description of the Related Art

Conventionally well-known heat pump water heaters using a naturalrefrigerant (e.g., carbon dioxide refrigerant) employ generally twokinds of heat exchangers, i.e., a heat radiator and a heat absorber. Thetwo heat exchangers use, as a heat transfer tube, refrigerant tubes forthe heat radiator and for the heat absorber, respectively.

In the heat pump water heaters, the heat radiator is also called“water-refrigerant heat exchanger” and uses also another heat transfertube (which is called “water tube for the heat radiator” or herein alsocalled simply “water tube”) for heat exchange to the refrigerant otherthan the above two heat transfer tubes. Fluid to flow inside the threeindividual heat transfer tubes (i.e., the water tube for the heatradiator, the refrigerant tube for the heat radiator and the refrigeranttube for the heat absorber) is different in kind, pressure and flowrate. Therefore, the technical specifications to be requiredrespectively for the heat transfer tubes are also different.

For example, as the water-refrigerant heat exchanger (herein calledsimply “heat exchanger”) used for the natural refrigerant heat pumpwater heater, there is a double tube heat exchanger that comprises twotubes of an outer tube through which water flows, and an inner tubethrough which a refrigerant flows. In such a double tube heat exchanger,in the inner tube through which a refrigerant flows, hole formation maybe caused by corrosion, which leads to a mixing of the water andrefrigerant. For this reason, a leak detection portion (a leak detectiontube with leak detection grooves) is often provided that detects wateror refrigerant leak to stop the apparatus (providing the leak detectiontube causes the heat exchanger to actually have a triple tubestructure).

On the other hand, the natural refrigerant heat pump water heater isused for boiling water during night, and has a small water flow ratewhich causes a laminar flow. For this reason, to enhance heat exchangerperformance, it is essential to enhance heat transfer performance of thewater tube that becomes a bottleneck.

JP-A-2004-360974 discloses a heat exchanger for the purpose ofenhancement in heat transfer performance, which comprises a first heattransfer tube and a plural-tube-helically-twisted second heat transfertube arranged in the first heat transfer tube. According toJP-A-2004-360974, the heat exchanger disclosed therein is small in waterpressure loss and in dissolution of a scale-forming constituent, andallows heat transfer promotion without using another heat transferpromotion component.

Also, JP-A-2002-228370 discloses a heat exchanger that comprises a watertube as its core tube and a refrigerant tube wound therearound, wherethe core tube is constructed from a plain tube, a corrugated tube or aninner-grooved tube, or by inserting a torsion sheet in the core tube.According to JP-A-2002-228370, the heat exchanger disclosed therein hasthe advantages of ease of fabrication and conveyance, enhancement ofheat exchange, reduction of cost, etc.

In the heat exchanger disclosed in JP-A-2004-360974, however, there arethe problems that the plural-tube-helically-twisting step itself iscomplicated and costly (twisting a hollow tube that tends to deform(collapse, break, etc.) is not so easy compared to twisting a solidwire), and that the treatment (structure) of the heat exchanger ends, inwhich the first heat transfer tube and the plural-tube second heattransfer tube are separated from each other, is complicated. There isalso the problem that, when the above-mentioned leak detection portionis provided, it is necessary to cause each of the second heat transfertube to comprise a double tube, which leads to an even higher cost.

Also, in JP-A-2002-228370, simply forming the core tube in a corrugatedshape or inserting the torsion sheet in the core tube may make nodesired heat transfer performance, and an increase in cost or pressureloss. Also, where an inner-grooved tube is used as the core tube, alaminar flow region by a small flow-rate has no effect caused by anincrease of heat transfer area even though the heat transfer areaincreases. Further, because of limitations on an inner-grooved tubefabrication method, it is difficult to form a shape change to cause aturbulence effect in a laminar flow region by the small flow-rate.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a heattransfer tube for a heat exchanger, capable of enhancing heat transferperformance of the heat exchanger when used for a small water-flow-ratein a natural refrigerant heat pump water heater, and a heat exchangerusing the heat transfer tube.

-   (1) According to a first aspect of the invention, a heat transfer    tube comprises:

a corrugated water tube to be used in a heat exchanger, and satisfying0.04≦Hc/OD, where Hc is a corrugated groove depth of the corrugated tubeand OD is a corrugation outside diameter thereof.

In the above invention (1), the following modifications and changes canbe made.

(i) 0.04≦Hc/OD≦0.1.

(ii) A twist angle βc defined between a corrugated groove of thecorrugated tube and a tube axis thereof is βc≧40°.

-   (2) According to a second aspect of the invention, a heat exchanger    comprises:

a heat transfer tube comprising a corrugated water tube to be used in aheat exchanger, and satisfying 0.04≦Hc/OD, where Hc is a corrugatedgroove depth of the corrugated tube and OD is a corrugation outsidediameter thereof.

In the above invention (2), the following modifications and changes canbe made.

(iii) The heat exchanger further comprises an outer tube providedoutside of the heat transfer tube that is used as an inner tube, theheat exchanger formed so that a refrigerant flows through an annularportion between the heat transfer tube and the outer tube.

(iv) The heat exchanger further comprises a plain tube sheathed on theheat transfer tube to form a leak detection portion, and an outer tubearranged outside of the plain tube, the heat exchanger formed so that arefrigerant flows through an annular portion between the plain tube andthe outer tube.

(v) The outer tube comprises a corrugated tube.

-   (3) According to a third aspect of the invention, a heat exchanger    comprises:

a heat transfer tube comprising a corrugated water tube to be used in aheat exchanger, and satisfying 0.04≦Hc/OD, where Hc is a corrugatedgroove depth of the corrugated tube and OD is a corrugation outsidediameter thereof; and

a refrigerant-conducting heat transfer tube wound around on the heattransfer tube.

ADVANTAGES OF THE INVENTION

According to the present invention, it is possible to provide a heattransfer tube for a heat exchanger that is capable of enhancing heattransfer performance of the heat exchanger when used for a smallwater-flow-rate in a natural refrigerant heat pump water heater, and aheat exchanger using the heat transfer tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments according to the invention will be explainedbelow referring to the drawings, wherein:

FIG. 1A is an explanatory diagram of an entire view showing structure ofa heat transfer tube in a first preferred embodiment according to thepresent invention;

FIG. 1B is an enlarged cross-sectional view in region A of FIG. 1A;

FIG. 2 is an explanatory diagram showing structure of a heat transfertube in a second preferred embodiment according to the presentinvention;

FIG. 3 is an explanatory diagram showing structure of a heat exchangerin a third preferred embodiment according to the present invention;

FIG. 4 is an explanatory diagram showing structure of a heat exchangerin a fourth preferred embodiment according to the present invention;

FIG. 5 is an explanatory diagram showing structure of a heat exchangerin a fifth preferred embodiment according to the present invention;

FIG. 6 is an explanatory diagram showing structure of a heat exchangerin a sixth preferred embodiment according to the present invention;

FIG. 7 is an explanatory diagram showing structure of a heat exchangerin a seventh preferred embodiment according to the present invention;

FIG. 8 is a diagram showing the comparison of heat transfer performanceof the corrugated heat transfer tube of the first embodiment (example1), of a plain tube (comparison example 1), and of an inner-grooved tube(comparison example 2);

FIG. 9 is a diagram showing the relationship between Hc/OD and heattransfer performance of the corrugated heat transfer tube, i.e., heattransfer performance ratio relative to a plain tube for Reynolds numberRe=1000;

FIG. 10 is a diagram showing the relationship between twist angle βc andheat transfer performance of the corrugated heat transfer tube, i.e.,heat transfer performance ratio relative to a plain tube for Reynoldsnumber Re=1000; and

FIG. 11 is a diagram showing the relationship between Hc/OD and frictioncoefficient of the corrugated heat transfer tube, i.e., tube frictioncoefficient ratio relative to a plain tube for Reynolds number Re=1000.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1A is an explanatory diagram of an entire view showing structure ofa heat transfer tube in a first preferred embodiment according to thepresent invention, and FIG. 1B is an enlarged cross-sectional view inregion A of FIG. 1A.

A heat transfer tube (a corrugated heat transfer tube) 10 in thisembodiment is formed of a one-thread corrugated tube (“one-thread” meansthat a number of a corrugated groove is one), and is used as a watertube that constitutes a heat exchanger (e.g., a water-refrigerant heatexchanger for a heat pump water heater), where a heat is exchangedbetween a water flowing inside the heat transfer tube 10 and arefrigerant flowing outside the heat transfer tube 10. The corrugatedtube generally refers to a tube with an undulating helical structure inits inner/outer surface.

Let the corrugated groove depth and corrugation outside diameter of thecorrugated heat transfer tube 10 in this embodiment be Hc and ODrespectively, Hc/OD, which represents the unevenness-to-outside-diameterratio of the corrugated heat transfer tube 10, is then large comparedwith the unevenness-to-outside-diameter ratio (=“groove depth/outsidediameter”) of a typical inner-grooved tube. The corrugated heat transfertube 10 satisfies 0.04≦Hc/OD, preferably 0.04≦Hc/OD≦0.1, more preferably0.04≦Hc/OD≦0.07, and can thereby have good heat transfer performance andlow pressure loss.

Also, let the angle which corrugated grooves 1 of the corrugated heattransfer tube 10 make with tube axis Ta thereof be a twist angle βc, βcis then desirably 40° or higher, more desirably 40°≦βc≦82°. This allowspromoting fluid turbulence which has crossed the unevenness. From theabove definition of the corrugated tube, the twist angle βc ranges0°<βc<90°.

Thickness Tw of an end region with a plain shape and corrugation pitchPc of the corrugated heat transfer tube 10 are not particularly limited,but may be 0.4 mm≦Tw≦1.7 mm and 3 mm≦Pc≦10 mm, respectively, forexample. Also, its material is not particularly limited, but may, fromthe point of view of thermal conductivity and mechanical strength, bepreferably copper or copper alloy, aluminum or aluminum alloy, or thelike.

Second Embodiment

FIG. 2 is an explanatory diagram showing structure of a heat transfertube in a second preferred embodiment according to the presentinvention.

While the heat transfer tube 10 in the first embodiment is formed fromthe one-thread corrugated tube, a heat transfer tube 20 in this secondembodiment is formed from a three-thread corrugated tube (“three-thread”means that a number of a corrugated groove is three), and is used as awater tube that constitutes a heat exchanger. The more the number ofthreads, the higher the fabrication rate, which therefore results in alarge cost merit.

Although the twist angle βc in three thread fabrication tends to besmaller than that of one thread fabrication, by reducing the spacingbetween the adjacent corrugated grooves 1, i.e., corrugation pitch Pc, atwist angle of 40° or higher, which is difficult to fabricate in theinner-grooved tube, can be realized.

Next, there is explained a heat exchanger equipped with the abovecorrugated heat transfer tube.

Third Embodiment

FIG. 3 is an explanatory diagram showing structure of a heat exchangerin a third preferred embodiment according to the present invention.

A heat exchanger (a double tube heat exchanger) 100 in this embodimentincludes an outer tube 11 provided outside of the heat transfer tube(e.g., corrugated heat transfer tube 10) in the above-describedembodiments that is used as an inner tube, where the heat exchanger isformed so that a refrigerant flows through an annular path between thecorrugated heat transfer tube 10 and the outer tube 11.

Fourth Embodiment

FIG. 4 is an explanatory diagram showing structure of a heat exchangerin a fourth preferred embodiment according to the present invention.

A heat exchanger (a triple tube heat exchanger) 200 in this embodimentincludes a leak detection tube 12 comprising a plain tube arranged incontact with the periphery of the heat transfer tube (e.g., corrugatedheat transfer tube 10) in the above-described embodiments that is usedas an inner tube, to form therearound a leak detection portion (a leakdetection grooves 13), and an outer tube 11 arranged outside of the leakdetection tube 12, where the heat exchanger is formed so that arefrigerant flows through an annular path between the leak detectiontube 12 and the outer tube 11.

Five and Sixth Embodiments

FIG. 5 is an explanatory diagram showing structure of a heat exchangerin a fifth preferred embodiment according to the present invention. FIG.6is also an explanatory diagram showing structure of a heat exchanger ina sixth preferred embodiment according to the present invention

Heat exchangers 300 and 400 shown in FIGS.5 and 6 use corrugated outertubes 21 in place of the outer tubes in the heat exchangers of FIGS.3and 4, respectively. This allows enhancement in flexibility for bendingof the heat exchanger.

Seventh Embodiment

FIG. 7 is an explanatory diagram showing structure of a heat exchangerin a seventh preferred embodiment according to the present invention.

A heat exchanger 500 in this embodiment is constructed by winding arefrigerant-conducting helical heat transfer tube 31 along thecorrugated grooves 1 of the heat transfer tube (e.g., corrugated heattransfer tube 10) in the above-described embodiments. The corrugatedgrooves 1 and heat transfer tube 31 may be securely brazed to eachother, if desired.

In the heat exchanger 500, heat is exchanged between a water flowinginside the heat transfer tube 10 and a refrigerant flowing inside thehelical heat transfer tube 31 in contact with the periphery of the heattransfer tube 10. Because the heat transfer tube 31 is wound along thecorrugated grooves 1, it is possible to increase effective contact area(effective heat transfer area) between the heat transfer tube 10 and theheat transfer tube 31.

Other Embodiments

As the embodiments of the present invention, besides the above-describedembodiments, there are various embodiments. For example, while theone-thread and three-thread corrugated heat transfer tubes have beenexplained, the corrugated heat transfer tube may comprise two, or fouror more threads. The one-to-three-thread corrugated heat transfer tubeis desirable in that it is easy to realize a high twist angle difficultto fabricate in the inner-grooved tube.

Advantages of the Embodiments

The embodiments of the present invention have the following advantages:

(1) In a prior-art heat transfer tube (a plain tube, an inner-groovedtube, etc.), there is the problem of very low heat transfer performancedue to a very small water-flow-rate in a water-refrigerant heatexchanger of a heat pump water heater, leading to a laminar flow in theheat transfer tube. Also, a prior-art heat transfer tube using acorrugated tube does not define unevenness-to-outside-diameter ratioHc/OD, and is indefinite in heat transfer performance effect. Incontrast to these, according to the corrugated heat transfer tube in thepresent embodiments, the unevenness-to-outside-diameter ratio Hc/OD canbe sufficiently large at low cost even compared to an inner-groovedtube, and the heat transfer performance can be substantially enhanced bythe turbulence effect of fluid crossing unevenness defined by thisHc/OD. Particularly, it is possible to realize twice or more theperformance compared to a plain tube, in a low Reynolds number Re range(e.g., 1000-5000, particularly 1000-3000) difficult to enhance theperformance in the prior-art product.

(2) According to the corrugated heat transfer tube in the presentembodiments, because the twist angle βc which the corrugated groovesmake with the tube axis can be 40° or higher, which is difficult to formin the inner-grooved tube, it is possible to increase the frequency offluid crossing unevenness, and thereby promote fluid turbulence effect.Also, by adjusting the relationship between the number of threads andthe corrugation pitch Pc, it is possible to make the twist angle βclarge at low cost compared to the inner-grooved tube, etc.

(3) According to the corrugated heat transfer tube in the above third tosixth embodiments, because it is possible to maximize enhancement in theheat transfer performance of the water tube and the heat transfer areaof the water tube relative to the refrigerant, the heat exchangerefficiency is enhanced. Further, according to the above third and fifthembodiments, it is possible to ensure enhancement in the heat transferperformance of the refrigerant in addition to the heat transferperformance of the water tube.

(4) It is possible to relatively easily provide a large leak detectionportion, in comparison to the inner-grooved tube. Specifically, althoughleak detection groove formation typically requires use of aninner-grooved tube with high fins as a leak detection tube, because thecorrugated heat transfer tube is used as the inner tube, it is possibleto make the leak detection grooves large (at low cost), and thereby usea plain tube as the leak detection tube 12.

(5) According to the above fifth and sixth embodiments, the corrugatedouter tube allows enhancement in flexibility for bending of the heatexchanger.

(6) According to the above seventh embodiment, because the outer tubethrough which a refrigerant flows is helically wound along thecorrugated grooves of the corrugated heat transfer tube, it is possibleto have flexibility for bending of the heat exchanger, and increaseeffective contact area (effective heat transfer area) between the outertube and the water tube (the heat transfer tube around which is wound bythe outer tube), compared to the case where the outer tube is woundaround a plain tube or an inner-grooved tube.

EXAMPLE 1

FIG. 8 is a diagram showing the comparison of heat transfer performance,in the laminar flow regions (small Reynolds number regions), of thecorrugated heat transfer tube of the first embodiment (example 1), of aplain tube (comparison example 1), and of an inner-grooved tube(comparison example 2). Table 1 below shows specifications of the testedcorrugated heat transfer tube and inner-grooved tube. The heat transfertubes all comprise phosphorus deoxidized copper, and have the outsidediameter (OD) of 9.52 mm. Here, the heat transfer performance is definedby dividing Nusselt number Nu by Prandtl Number Pr raised to the powerof 0.4 (Nu/Pr^(0.4), the same applies to examples below), to cancel theaffects of fluid properties. Also, comparison is made for Reynoldsnumbers Re=1000, 2000, and 3000 that correspond to water flow amountsactually used in a heat pump water heater.

TABLE 1 Sample-tube specifications Example 1 Hc/OD Twist angle βc No. ofthreads Corrugated tube 0.1 81° 1 Comparison Groove depth/ Twist angleNo. of grooves example 2 outside diameter Inner-grooved tube 0.03 35° 40

As shown in FIG. 8, it is revealed that, in evaluated Reynolds-numberregions, the inner-grooved tube (comparison example 2) and plain tube(comparison example 1) have substantially the same heat transferperformance, whereas the corrugated heat transfer tube 10 has thesubstantially enhanced heat transfer performance of 3 times or morethose of the inner-grooved tube and the plain tube.

EXAMPLE 2

FIG. 9 is a diagram showing the relationship between Hc/OD and heattransfer performance of the corrugated heat transfer tube, i.e., heattransfer performance ratio relative to a plain tube for Reynolds numberRe=1000. Both of the twist angle βc and the number of threads of thecorrugated heat transfer tube are the same as in example 1 (Table 1).Also, as shown in FIG. 8, because the heat transfer performance of theinner-grooved tube is on the same order as that of the plain tube inthis flow rate region, the heat transfer performance of the corrugatedheat transfer tube is compared with that of the plain tube.

As shown in FIG. 9, it is revealed that, at less than 0.04 of Hc/OD, theheat transfer performance drops sharply. Thus, it is desirable that0.04≦Hc/OD.

EXAMPLE 3

FIG. 10 is a diagram showing the relationship between the twist angle βcand the heat transfer performance of the corrugated heat transfer tube,i.e., heat transfer performance ratio relative to a plain tube forReynolds number Re=1000. Both of the Hc/OD and the number of threads ofthe corrugated heat transfer tube are the same as in example 1 (Table1). Also, as shown in FIG. 8, because the heat transfer performance ofthe inner-grooved tube is on the same order as that of the plain tube inthis flow rate region, the heat transfer performance of the corrugatedheat transfer tube is compared with that of the plain tube.

As shown in FIG. 10, it is found that, for Hc/OD=0.1, the heat transferperformance of the corrugated heat transfer tube is higher by the orderof 1.5 times that of the plain tube even in the event of a small twistangle βc (βc=35°, for example). In addition, it is clarified that, bymaking the twist angle βc equal to or higher than 40°, the heat transferperformance of the corrugated heat transfer tube can be enhanced totwice or more that of the plain tube.

EXAMPLE 4

FIG. 11 is a diagram showing the relationship between Hc/OD and frictioncoefficient of the corrugated heat transfer tube, i.e., tube frictioncoefficient ratio relative to a plain tube for Reynolds number Re=1000.Here, the tube friction coefficient refers to a dimensionless number λdefined by the relation of “ΔP=λ×L/de×(ρv²)/2”, and can be regarded asan indicator of pressure loss where the affects of flow passage area,fluid flow rate, etc. are canceled. ΔP is the pressure loss of the heattransfer tube, L is the length of the heat transfer tube, de is theequivalent diameter (4×flow passage area/wetted perimeter) of the heattransfer tube, ρ is the fluid density, and v is the fluid flow rate.Both of the twist angle βc and the number of threads of the corrugatedheat transfer tube are the same as in example 1 (Table 1). Also, asshown in FIG. 8, because the heat transfer performance of theinner-grooved tube is on the same order as that of the plain tube inthis flow rate region, the tube friction coefficient of the corrugatedheat transfer tube is compared with that of the plain tube.

As shown in FIG. 11, it is found that, at less than 0.04 of Hc/OD, thetube friction coefficient ratio drops sharply as in the heat transferperformance ratio, making turbulence promotion impossible. On the otherhand, for 0.04 or higher Hc/OD, the tube friction coefficient ratio(i.e., pressure loss) continues to increase. Further, it is found that,beyond 0.1 of Hc/OD (0.1<Hc/OD), the tube friction coefficient ratioexceeds the heat transfer performance ratio (see FIG. 9). For example,at Hc/OD=1.1, the heat transfer performance ratio is 4.3, whereas thetube friction coefficient ratio is 4.5. It is therefore desirable that0.04≦Hc/OD≦0.1, which makes it possible to provide a high-performancecorrugated heat transfer tube with low pressure loss.

Although the invention has been described with respect to the specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A heat transfer tube, comprising: a corrugated water tube to be usedin a heat exchanger, and satisfying 0.04≦Hc/OD, where Hc is a corrugatedgroove depth of the corrugated tube and OD is a corrugation outsidediameter thereof.
 2. The heat transfer tube according to claim 1,wherein: 0.04≦Hc/OD≦0.1.
 3. The heat transfer tube according to claim 1,wherein: a twist angle βc defined between a corrugated groove of thecorrugated tube and a tube axis thereof is βc≧40°.
 4. A heat exchanger,comprising: a heat transfer tube comprising a corrugated water tube tobe used in a heat exchanger, and satisfying 0.04≦Hc/OD, where Hc is acorrugated groove depth of the corrugated tube and OD is a corrugationoutside diameter thereof.
 5. The heat exchanger according to claim 4,further comprising: an outer tube provided outside of the heat transfertube that is used as an inner tube, the heat exchanger formed so that arefrigerant flows through an annular portion between the heat transfertube and the outer tube.
 6. The heat exchanger according to claim 4,further comprising: a plain tube sheathed on the heat transfer tube toform a leak detection portion; and an outer tube arranged outside of theplain tube, the heat exchanger formed so that a refrigerant flowsthrough an annular portion between the plain tube and the outer tube. 7.The heat exchanger according to claim 5, wherein: the outer tubecomprises a corrugated tube.
 8. The heat exchanger according to claim 6,wherein: the outer tube comprises a corrugated tube.
 9. A heatexchanger, comprising: a heat transfer tube comprising a corrugatedwater tube to be used in a heat exchanger, and satisfying 0.04≦Hc/OD,where Hc is a corrugated groove depth of the corrugated tube and OD is acorrugation outside diameter thereof; and a refrigerant-conducting heattransfer tube wound around on the heat transfer tube.